There is an effort in the automotive industry to adapt the operating state of the engine in an optimum manner, during driving, to the current driving state of the motor vehicle in order to reduce emissions. This can be accomplished, for example, if the transmission has a large number of gears. In this case, selection is performed in such a way that the engine speed is held as low as possible, even at high driving speeds. Another aim is to keep down the masses which corotate under no load in order thereby to effect a further reduction in emissions.
In a conventional design, a large number of gears necessarily implies a relatively large number of transmission gearwheels and therefore a relatively large overall length, although this is not desired in modern motor vehicle engineering, as well as a relatively high weight of the transmission and relatively high associated power losses.
Known transmissions of the type stated at the outset (WO 2012/084250 A1; DE 10 2013 216 387 A1) are therefore already taking a step in the desired direction by providing the possibility of the power flowing both via a gearwheel stage associated with the first transmission input shaft and via a gearwheel stage associated with the second transmission input shaft.
In the case of such dual clutch transmissions, the transmission input shafts are designed as an inner shaft and an outer shaft, wherein the inner shaft and the outer shaft can be driven selectively. The additionally selected gears can then be routed along an indirect path which runs via gearwheels that are arranged on the respective undriven transmission input shaft.
These known dual clutch transmissions, by means of which it is possible, in addition to the existing gearwheel pairs and the gears that result therefrom, to select further gears, require hollow shaft devices for this purpose on the intermediate shafts and, in some cases, also on the transmission input shafts, on which hollow shaft devices in each case at least two gearwheels are arranged, wherein the hollow shafts corotate freely on the transmission input shafts and intermediate shafts but can be connected thereto for conjoint rotation when required.
The use of such hollow shaft arrangements represents not only a high additional weight contribution but also leads to an increase in overall lengths and overall heights and to poorer efficiency of the transmission.
It is therefore the underlying object of the invention to provide a dual clutch transmission which allows a large number of gears without enlarging the dimensions of the transmission and without significantly increasing the weight thereof.